The most frequently used methods are listed first. This does not necessarily make them the best methods, just the least rare.
Mass revives: Mostly only PVP servers have mass revives turned on. Whenever a mass revive happens you will get the option to come back to life and return to your spawn.
Revive ability: Players who have learned or been taught the revive ability can revive 1 player every few years. Dead people can not use revive so you must convince a living person to revive you. I also do not think alts are able to use revive on you. This ability is used by getting next to the person who is going to be revived, then face them and use the ability.
Absorbing people can bring you back to life. The requirements are: 1) Their base bp must be equal or greater than yours. 2) They must not have death regeneration. It takes absorbing only 1 living person to bring you back to life this way.
Reincarnation. This ability can be learned and used to reincarnate yourself or others. Only a living person can use reincarnate on someone. A reincarnated player is a 'new person' in roleplaying. Your "available potential" will be lowered to 1%, which means you will be at 1% BP, it is like being forced to be powered down. Available potential is easily retrained back to 100% within about 10 minutes. Using training methods with higher bp gains regains your hidden potential faster. You do not lose any actual BP.
Demon Altar: Somewhere in the bottom right quadrant of hell is an Altar, if you get next to it and click it, it will allow you to come back to life but only if you accept to become a vampire. Becoming a vampire is no big deal really, you get some extra power, but must bite people once in a while or your hunger reaches 100% and you die again.
Cloning: This involves 2 methods
1) Create a cloning tank thru science. This is very unpopular. Place the cloning tank in a safehouse that you created and set it to clone you. When you die it will revive you into a clone body. The clone will lose a little stats since it is not the original you.
2) This method is far more popular. Use a DNA Container from science to take the DNA of yourself or someone else, then take that DNA to a Genetics Computer to create a clone of that person and then do a brain transplant to go into that new living body. There will be a slight stat loss compared to the original person the DNA belongs to.
Android bodies: You can custom build an android body using Robotics Tools and Cybernetics Computer then when your ready you can use Genetics Computer to put your mind in the Android body. Androids are not technicly "alive" but they aren't "dead" either, and they have all the power of a living person.